I don't work in this area, but think about all of the variables that go into warhead maintenance. Your single supercomputer simulation can show that the warhea

I don't work in this area, but think about all of the variables that go into warhead maintenance. Your single supercomputer simulation can show that the warhead should work as-designed, but what will happen after the Plutonium pit has been sitting for a decade, slowly decaying? Will satisfactory implosion still happen if storage conditions slightly change the performance of the conventional explosives? Since modern warheads are all fusion-type warheads, there's also the fusion stage to consider with even more highly classified top-secret sauce. It appears that the conditions for fusion are triggered by radiation pressure, and that likely makes things even more complicated. Now, you need not just a successful supercritical fission event, but one of the right shape(?), timing, and interaction with other secret-sauce materials that might have their own degradation curves. So, rather than simulate one design, now you need to simulate hundreds to thousands to explore the full decay-over-time space. Getting the answer wrong means either very expensive premature warhead refurbishments or a nuclear stockpile that wouldn't work properly.